Wettability Effect on Evaporation Dynamics and Crystalline Patterns of

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Wettability Effect on Evaporation Dynamics and Crystalline Patterns of Sessile Saline Droplets Xin Zhong, Junheng Ren, and Fei Duan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03690 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

Wettability Effect on Evaporation Dynamics and Crystalline Patterns of Sessile Saline Droplets Xin Zhong, Junheng Ren, and Fei Duan∗ School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798 E-mail: [email protected]

Phone: +65 67905510. Fax: +65 67924062

∗ To

whom correspondence should be addressed

1 ACS Paragon Plus Environment


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Abstract The evaporative dynamics and crystalline patterns from sessile saline droplets on various substrates are experimentally investigated. On the silicon wafer and polymethyl methacrylate (PMMA) plate, the saline droplets exhibit unique evaporative dynamics such that the contact angle keeps increasing for a lasting period. Such an enlargement in contact angle is attenuated at a higher salt concentration. Interestingly, the onset of precipitation is almost overlapped with the end of contact angle enlargement when the contact angle reaches its apex. The lower wettability and the smaller pinning effect of silicon wafer and PMMA result in the morphology of crystalline cubes at the droplet center. On the soda lemon glass, the high wettability and lifetime pinning stage of the droplet lead to spherical profiles of precipitation. The crystalline deposit depends on the salt concentration on soda lemon glass such that it is comprised of exterior cracked layers of salt and interior separated small cubes for low salt concentrations, while large crystalline chunks stay near the droplet rim for high salt concentrations.

Introduction Crystallization of salts or other materials in supersaturated solutions has been intensively investigated due to its practical significance in pharmaceutical purification, salt manufacturing, seawater purification, cosmetic production, deicing and so on. 1–4 Mechanisms of crystallization have been mainly probed in electrolyte solutions without evaporation. Studies stressed on precipitation and crystallization from evaporating sessile droplets, however, are far less especially when compared with the active domain of colloidal sessile droplets. 5–16 The more complex profile of a sessile droplet characterized by the three-phase contact line and the curved liquid-vapor interface as compared to a easy solution configuration brings additional factors that could affect the precipitation process. The higher evaporation flux in the vicinity of the contact line can induce outward flows which result in heterogenous distribution of ions and the associated supersaturation degree. The curved liquid-vapor could limit the growth and vary the motion of precipitation. The complexity caused by the multi-factors in respect to evaporation, bulk flow and wettability are 2 ACS Paragon Plus Environment

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therefore expected to significantly vary crystallization in sessile droplets. So far crystalline of salts from drying saline droplets have been investigated in a number of studies mainly focused on nucleation mechanisms and the dependence of precipitation profile on solid surface properties, salt concentration, etc. 17–23 Kaya, et al. investigated the effects of polyelectrolyte concentration of drops and the surrounding humidity on the final salt crystallization, which exhibited profiles of concentric rings, needle-like and chain-like structures. 19 Takhistov, et al. investigated the crystal stains from microliter droplets on both hydrophilic and hydrophobic substrates. Concentric rings of salts were formed on hydrophilic surfaces while crystalline was produced on hydrophobic surfaces. 17 Shahidzadeh, et al. also examined the evaporation and stain structures on various substrates by adopting two types of salts, sodium chloride (NaCl) and calcium sulfate (CaSO4 ), with different crystalline structures and precipitation pathways. The crystalline pattern in a variety was concluded to be controlled by the interfacial properties of the emerging crystalline and the number of crystals generated. 20 The study of crystallization from saline droplet is propelled by Shin, et al. through producing three-dimensional salt structures from droplets with high aspect ratio. A rich variety of three-dimensional crystalline deposits were observed. 21 The aforementioned studies are mainly concerned on the crystalline morphologies in a variety from the evaporation of sessile droplets of saline solutions. The variation in the evaporative regimes of sessile droplets caused by dissolved salt or other materials, however, has been scarcely focused on. Understanding the role of salt in varying droplet evaporation could provide a new path for controlling contact line dynamics, solution wettability and pinning-to-depinning transition. Besides, it is crucial to systematically examine the interrelation between droplet behavior and salt precipitation, particularly to answer the fundamental questions of how exactly the droplet dynamics and the bulk flow affect the nucleation and motion of crystalline, and how do surface properties like wettability and pinning effect determine the evaporative dynamics and the associated precipitation. The knowledge of droplet dynamics dependent salt crystalline are vital for controlling and designing compound stains particularly in biological and medical science. Motivated by these,



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here we employe three types of substrates with different wettability and roughness to investigate the evaporative regimes and crystallization of aqueous droplets with various NaCl concentrations. The substrates used include two types that allow contact line receding and one type with high pinning effect. The initial salt concentrations are kept below the saturation concentration (26.4%) so precipitation occurs in the middle of evaporation. Our study demonstrates that the saline droplets allowed to shrink during drying exhibited unique evaporative dynamics. We also showed that how the crystal stains and their movement are governed by the evaporative dynamics and the wettability of droplets, which are determined by solid surface properties.

Experimental Methods The homogeneous saline solution samples were prepared by dissolving sodium chloride powders (NaCl, Sigma Aldrich, >99%) in nano-filtered water with resistivity at 18.2 MΩ-cm. The initial concentration Csalt of the NaCl solutions were 0%, 2.5%, 5%, 10%, 20% and 25%, which were lower than the saturation concentration at roughly Csat =26.4%. 24 Evaporation substrates were brand new silicon wafers (Latech Scientific Supply Pte. Ltd.), PMMA (POLY-A, cast acrylic sheet) and soda lemon glass (Latech Scientific Supply Pte. Ltd.) which were thoroughly cleaned prior to be used. The average roughness and wettability reflected by the contact angle of deionized water droplet are listed in Table 1 for each type of substrate. Table 1: The averaged roughness of and the contact angle of deionized water droplet on the substrates of silicon wafer, PMMA and soda lemon glass. Silicon Wafer PMMA Roughness (µ m) 0.1 0.05 ◦ Contact Angle ( ) 39.6 67.0

Soda Lemon Glass 0.2 20.1

The schematics of the experimental setup is shown in Figure 1. Droplet evaporation process and the formed crystalline precipitation after evaporation completed were simultaneously visualized by the optical microscope (Nikon Eclipse LV100ND) with the brightfield schema from top view and


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Figure 1: Schematics of the experimental configuration. Droplet evaporation is simultaneously visualized by the microscope from top view and the fast camera from side view. the HiSpec 2 high-speed camera from side view. Snapshots of droplet profile from the side-view were post-processed by the program ImageJ to extract quantitative information including contact angle and baseline length. 25 During droplet evaporation, the loss of water led by evaporation resulted in an increase in salt concentration and the droplet could reach a state of supersaturation, under which the formation of crystalline could be initiated. Herein the instant salt concentration which varies during evaporation is denoted by Csalt (t). Since the crystalline was produced when the instant salt concentration Csalt (t) was higher than the saturation concentration Csat for all the droplets in our experiment, we use the relative supersaturation S = Csalt (t)/Csat to reflect the degree of supersaturation at the onset of crystallization. The moment when crystalline starts to appear is defined as the "precipitation time" tp . Crystalline can grow to a degree that it defects the liquidvapor interface or the three phase contact line. But still, we report the data of contact angle and baseline length both prior to and after the deformation of droplet spherical profile for the sake of providing full spectrum of information. The error bars for some parameters shown in figures indicate the standard deviations obtained from repeated tests at each experimental condition. Droplets were evaporating in an open condition with room temperature and humidity maintained at 22±1 ◦ C and 55±5%.



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Results and Discussion The evaporation dynamics and the resulting crystallization patterns are investigated for aqueous droplets on a variety of surfaces with different wetting properties and roughness. To provide a general view of the saline droplets evaporation, snapshots taken simultaneously from side view and top view for the droplets with Csalt at 10% on the three substrates are shown in Figure 2. It can be seen that the droplet is the most flattened on the soda lemon glass (Figure 2 (c)), less flattened on the silicon wafer (Figure 2 (a)), and least flattened on the PMMA plate (Figure 2 (b)). It implies that the soda lemon glass is more hydrophilic than the silicon wafer and the PMMA plates. Besides, the evaporation regimes of the droplets on different substrates are various as well. On the silicon wafer and the PMMA substrates, the droplets exhibit evident depinning during drying; while the droplet remains pinned for most the lifetime on the soda lemon glass. The corresponding droplet lifetime with Csalt at 10% lasts 60.7 minutes for the PMMA, 32.1 minutes for the silicon wafer, and 14.9 minutes for the soda lemon glass. Longer pinning stages in conjunction with larger contacting areas of the droplets result in more rapid evaporation, and vice versa. These NaCl solution droplets with the intermediate Csalt produce crystalline morphologies in a variety after evaporation completed. Figure 3 presents the crystalline patterns from saline droplets with Csalt ranging from 2.5% to 25% on the three substrates. The crystalline profile is fairly alike for silicon wafer and PMMA plate. As shown in Figure 3 (a, b), the crystalline is either a monocube or accumulated crystals with jagged edge regardless of the salt concentration, except that the crystalline is larger at a higher Csalt . On the soda lemon glass wafer, however, it as a whole is a spherical profile comprised of isolated cubic chunks and cracked thin layer of crystalline. Such a pattern basically matches the droplet initial contact area with the substrate. Therefore, from the above general view it is seen that both the stain morphology and evaporative dynamics are alike for silicon wafer and PMMA while different from the case of soda lemon glass. So the following former two sections are stressed on evaporating saline droplets on silicon wafer & PMMA, and on soda lemon glass respectively, followed by the last section exclusively comparing the motion and crystalline morphologies on various substrates. 6 ACS Paragon Plus Environment

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0%

25%

50%

75%

100%

(a)

(b)

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Figure 2: The snapshots taken at 0%, 25%, 50%, 75% and 100% of the lifetimes of droplets with Csalt at 10% on the (a) silicon wafer, (b) PMMA and (c) soda lemon glass substrates. The scale bar indicates 1 mm.



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(a) Si

(b) PMMA

(c) Glass

2.5%

5%

10%

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Figure 3: Crystalline patterns from droplets with Csalt at 2.5%, 5%, 10%, 20% and 25% on (a) silicon wafer, (b) PMMA and (c) soda lemon glass. The scale bar indicates 1 mm.


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Saline Droplet Evaporation on Silicon Wafer and PMMA Plate The evolution of contact angle and baseline length are shown in Figure 4 (a1, a2) for saline droplets on silicon wafer. The droplets at various Csalt show an initial pinning stage, reflected by the reducing contact angle and unvaried baseline length. Afterwards the droplets begin to depin. In this stage it is interesting to find that for Csalt from 2.5% to 20%, the contact angle exhibits a relatively long period of increase before re-declining till the end of drying. The corresponding baseline reduces accordingly, and the reducing slope is attenuated at a higher Csalt (Figure 4 (a2)). Later, the contact angle reaches the apex and starts to reduce again, and at the same moment the baseline starts to decrease more rapidly, indicating that the droplets enter the ultimate fast evaporation phase. At 25% of Csalt , the baseline keeps nearly constant for approximately 70% of the droplet lifetime, and the contact angle remains declining without any enlargement emerging in the other saline droplets. Besides, the droplets with the same Csalt show fairly analogous evaporative regimes on the PMMA plate, as shown in Figure 4 (b1, b2). The contact angle exhibits various enlargement at different Csalt , and the baseline has a steeper declining trend at a lower Csalt . The increase of contact angle in the middle of evaporation reflects the role of salt in modifying the evaporative dynamics as the droplet solution is concentrated during drying. To analyze the dependence of contact angle increment on Csalt , we plot the increment ∆θ and the increasing rate kθ of contact angle in respect to Csalt on silicon wafer and PMMA plate in Figure 5, respectively. The inset of Figure 5 (a) as an example shows that the contact angle increment ∆θ is the difference between the two turning points of the contact angle curve of the droplet with Csalt at 2.5% (Figure 4 (a1)), and the increasing rate of contact angle kθ is obtained by applying the best-fitting line on the section between the two turning points. From Figure 5 it can be seen that ∆θ decreases monotonously with an increase in Csalt , suggesting that the depinning-induced contact angle enlargement is weakened at a higher salt concentration. The increasing rate kθ shows a similar trend that the contact angle increases slower at a higher initial Csalt . These data implies that a droplet with a lower initial Csalt is farther from its equilibrium state. Droplet shrinkage during evaporation represents that it is under an non-equilibrium state, and 9 ACS Paragon Plus Environment


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Figure 4: (a1) contact angle and (a2) baseline length for droplets with Csalt at 0%, 2.5%, 5%, 10%, 20% and 25% on silicon wafer. (b1) Contact angle and (b2) baseline length for droplets with Csalt at 0%, 2.5%, 5%, 10%, 20% and 25% on PMMA plate.


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it is driven toward an equilibrium one. Based on Young’s equation, the discrepancy between the dynamic contact angle θ and the spontaneous equilibrium contact angle θE (t) at the same time leads to the contact line force Fc that drives the depinning behavior. Fc is expressed as Fc = γLV (cosθ − cosθE (t))

(1)

The equilibrium contact angle θE (t) describes the contact angle of a sessile droplet as it is at equilibrium under ideal conditions. It should vary with the proceeding of evaporation since the salt concentration is kept increasing led by the loss of water component. Besides, it is normally reckoned that the equilibrium contact angle can be reflected by the initial contact angle of a sessile droplet in actual cases. As shown by the initial contact angle at various Csalt listed in Table 2, the initial contact angle θ0 is larger at a higher Csalt , which means that a saline droplet has a raising equilibrium contact angle reflecting the instant equilibrium with an increase of the salt concentration. Furthermore, the dynamic contact angle θ reduces during the initial pinning stage as shown in Figure 4 (a1, b1). Therefore, (cosθ − cosθE (t)) increases with the proceeding of evaporation. Since the liquid-vapor surface tension γLV is also increased along with the concentrating solution, the contact force is further enhanced, leading to the observed contact line receding induced contact angle enlargement. Table 2: The initial contact angle θ0 at various Csalt of droplets on silicon wafer (S) and PMMA (P). Csalt % θ0 (S) θ0 (P) 0 39.6 67.0 2.5 40.8 67.9 5 40.6 74.0 10 44.3 74.1 20 43.8 74.7 25 48.2 76.4 The contact angle θ , however, is unable to increase infinitely. When the saline solution is concentrated to a certain supersaturation state, crystalline formation would start and the rise of Csalt would be terminated. As a result, the contact angle is anticipated to stop increasing and could 11 ACS Paragon Plus Environment


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Figure 5: (a) contact angle increment ∆θ and (b) increasing rate of dynamic contact angle kθ with Csalt at 0%, 2.5%, 5%, 10%, 20% and 25% on silicon wafer and PMMA plates. The inset of (a) indicates the ∆θ and the kθ obtained from the contact angle curve of the droplet with Csalt at 2.5% on silicon wafer.


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have reached the apex. It could account for the intensified enlargement and the increasing rate of contact angle at a lower salt concentration shown in Figure 5, since it has a larger gap for the droplet to reach the moment of salt precipitation. To confirm it, we plot the time tapex when the apex contact angle is reached and the time of precipitation tp in Figure 6 for both silicon wafer and PMMA plate. Both the times are normalized to the corresponding droplet lifetime. Surprisingly, the time tapex for contact angle apex θapex is fairly close to the onset of crystalline precipitation tp for both substrates. The predicted time of precipitation tp′ normalized to droplet lifetime at which supersaturation concentration Csat is arrived is also plotted for comparison in Figure 6. The predicted precipitation time tp′ is obtained based on the evolution of droplet volume. Since that the Bond number Bo for the droplets in our study is one order of magnitude smaller than unity, the droplets can be seen as spherical-shaped. Therefore, the instantaneous volume of the droplet during evaporation V (t) can be expressed as 26

π r3 V (t) = 3

(

2 − 3cosθ + cos3 θ sin3 θ

) (2)

V (t) is a function of the instantaneous radius r and dynamic contact angle θ , and the two parameters are time-dependent. The time at which the volume is reduced to make Csalt equal to Csat (26.4%) is reckoned as the predicted onset of precipitation. The predicted time of precipitation tp′ is smaller than the actual onset of precipitation tp for all the saline droplets on both substrates, implying that the droplets have reached supersaturation prior to precipitation. The precipitation time is even more underestimated that tp′ is greatly smaller than t p particularly at a high Csalt , eg. 20% and 25%. Such an underestimation occurs for both silicon wafer and PMMA plate. To reflect the dependence of supersaturation at which precipitation occurs on Csalt , the relative supersaturation S for silicon wafer and PMMA plate are presented in the insets of Figure 6 (a, b) respectively. It can be seen that basically S has an upward trend with Csalt such that it is greatly larger with Csalt at 20% and 25%. It indicates that precipitation occurs at a higher supersaturation degree when there are more salts in the droplet solution. Therefore,



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15 Csalt (%)

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Figure 6: The normalized precipitation time tp , the normalized predicted precipitation time tp′ and the normalized time at contact angle apex tapex to the lifetime of droplet at intermediate Csalt on (a) silicon wafer and (b) PMMA plate. The inset is the relative supersaturation S of droplet with intermediate Csalt on (a) silicon wafer and (b) PMMA plate, respectively.


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droplets with higher Csalt are capable of reaching and staying at higher supersaturation. The delay of precipitation in high Csalt droplet could be correlated with the non-uniformity of the ions of NaCl within the droplet region. 1.0

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Figure 7: The Peclet number Pe of the droplets with intermediate Csalt on (a) silicon wafer and (b) PMMA plates. The inset is the Peclet number at 0 s, 420 s and 780 s at intermediate Csalt on (a) silicon wafer and (b) PMMA plate, respetively. To characterize the driving force for the heterogeneity of electrolyte distribution within droplet, Peclet number Pe reflecting the ratio of the rate of advection of ions to the rate of diffusion is introduced herein for the droplets with intermediate Csalt . Pe is expressed by Pe = Ur/Di , where U is the typical flow velocity inside the droplet and Di is the diffusion coefficient of cations Na+. 15 ACS Paragon Plus Environment


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U is determined by the evaporation flux J and the spherical cap area A(t)

U = J/A(t)

(3)

J = −dV /dt

(4)

with J expressed as

Di for Na+ can be described by the formula 20 DNa+ = −0.1097 × 10−9 m(t) + 1.324 × 10−9

(5)

where m(t) is the concentration in (mol/kg) in the drop at the time t since the concentration increases up to 10 M during evaporation before crystals precipitate. 20 Increasing the salt concentration changes the viscosity of the solution and therefore modifies the diffusion coefficient. It can be seen from Figure 7 that for all the Csalt , Pe is smaller than unity on both silicon wafer and PMMA plate, indicating the dominant role of diffusion over convection and thus the relatively uniform distribution of ions in the droplet. The insets of Figure 7 (a, b) illustrate Pe at time 0 s, 420 s and 780 s for the droplets with different Csalt on silicon wafer and PMMA plate, respectively. Although Pe remains smaller than unity for most the droplet lifetime at various Csalt and on both the substrates, we found the locale where precipitation firstly occurs is always in the vicinity of the contact line, suggesting that the ions concentration near the contact line should be higher than elsewhere. At 25% of Csalt , Pe is smaller than the ones at other concentrations for both the substrates, so the heterogeneous distribution of ions characterized by its accumulation at the periphery is expected to be weakened. Based on the fact that the diffusion coefficient DNa+ is smaller at a higher Csalt due to enhanced viscosity, the advective flow should be attenuated at a high Csalt . Therefore, the distribution of NaCl ions is expected to be more homogeneous in the droplet with a high Csalt , which could partially account for the late appearance of salt precipitation. Another reason accounting for the late appearance of precipitation at high Csalt is associated with the droplet


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dynamics during evaporation. It is known that supersaturation is a metastable state and it can be easily terminated by introducing stimulus from interior, such as enhancing the interaction among ions, or from exterior, like imposing vibrations on the solution or adding nuclei for crystallization. The high supersaturation degree at 25% of Csalt is presumably due to the longer pinning stage of the droplet on both the substrates, in contrary to the intense and lasting depinning droplets with the other Csalt (see Figure 4 (a2, b2)). The receding contact line acts like a stimulus which destabilizes the droplet solution and hence could initiate the crystallization. At a pinning state, however, the interaction among ions is mainly determined by diffusion, as reflected by the small Pe, and therefore the droplet is able to maintain a longer period of supersaturation.

Saline Droplet Evaporation on Soda Lemon Glass The droplet dynamics on the soda lemon glass is distinctly different from that on the silicon wafer and PMMA plate. As illustrated in Figure 8, the contact angle keeps reducing and the baseline length maintains almost constant until the ultimate plunging at the intermediate Csalt , reflecting that the droplet remains pinned for nearly the lifetime. The higher initial contact angle and the smaller baseline at a higher Csalt suggest the attenuation of droplet wettability led by NaCl. The pinned droplets on soda lemon glass had no increase in the dynamic contact angle during the drying, indicating that the pinning effect is sufficient to overcome the concentrating solution induced depinning effect. Therefore, different from the case of silicon wafer and PMMA plate, the droplet does not have the time tapex at which contact angle reaches the apex, but we solely compare the actual precipitation time tp with the predicted one tp′ . It can be seen from Figure 9 that at all Csalt precipitation takes place when saline solution is supersaturated. The supersaturation degree S against Csalt is shown in the inset of Figure 9 (a) which has little variation and stays between 1.3 to 1.6, indicating that the precipitation occurs at similar supersaturation for various Csalt . Since the saline droplets keep pinned for nearly the lifetimes at the soda lemon glass regardless of Csalt , the salt precipitation should be determined by diffusion or advection instead of the disturbance from contact line dynamics. To elucidate the phenomenon, the Peclet number Pe as a function of time 17 ACS Paragon Plus Environment


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2.5

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for each Csalt is shown in Figure 9 (b). Pe is insensitive to Csalt that it varies within 0.2 to 0.3, suggesting that the competence between diffusion and convection is alike regardless of the initial salt concentration. Therefore, the associated transport and accumulation of the salt ions in the droplet region could be analogous as well for different Csalt , resulting in crystallization at similar extents of supersaturation. 1.0 (a)

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Figure 9: (a) The normalized precipitation time tp and the normalized predicted precipitation time tp′ to the lifetime of droplet at intermediate Csalt on soda lemon glass. The inset is relative supersaturation S for soda lemon glass, silicon wafer and PMMA plate. (b) The Peclet number Pe of the droplets with intermediate Csalt on soda lemon glass. The inset is the Peclet number at 0 s, 180 s and 420 s. It is worth noticing that the supersaturation S is higher on soda lemon glass than on silicon 19 ACS Paragon Plus Environment


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wafer and PMMA plate, as presented in the inset of Figure 9 (a). S on silicon wafer and PMMA are evidently below that on soda lemon glass. This again is owing to the different evaporative regimes on various substrates. Despite that the high wettability of the droplet on soda lemon glass leads to stronger advective flows and the associated accumulation of ions near the perimeter, its lifetime pinning makes the droplet remain under a relatively static state barely with external stimulus. By comparing the supersaturation degree on the three types of substrates, it can be concluded that contact line dynamics is vital to inspire the onset of precipitation as the droplet has reached a supersaturation state.

Motion and Morphologies of Crystalline on Various Substrates The distinctly different crystalline profiles on silicon wafer & PMMA and on soda lemon glass indicate the dependence of crystalline on surface properties. It is found that the pinning effect and surface wettability are the crucial factors that determine the motion and morphology of precipitation. For droplets on either the silicon wafer or the PMMA plate, the site at which precipitation initially occurs is near the contact line region, as presented in Figure 10. The crystal firstly formed at the liquid-vapor interface grows to occupy the confined space between the free surface and the solid-liquid interface. 20 The enlarging crystal could lead to a distortion of the free surface which exerts capillary forces that push the crystal cube toward the droplet center. Herein the inward motion of the crystals is ensured by the the lower wettability and weak pinning effect of silicon wafer and PMMA plate. The lower wettability of the solid surface leads to an initial large thickness of the droplet. Later, the sustainable receding phase in the middle of drying provides a even thicker region for the upcoming growth of precipitation. As a result, crystalline forming near the perimeter would be less possible to cause a sever deformation of the liquid-vapor interface, as shown by the snapshots representing the inward motion of the crystal cube for silicon wafer from side view in Figure 11 (a). The small intrusion of the crystal upon the liquid-vapor interface suggests little effect on capillary forces since the crystalline is still mainly confined in the solution region. An addition reason could be the smaller Pe as compared to the counterpart of soda lemon 20 ACS Paragon Plus Environment

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glass suggests a milder accumulation of salt ions near the droplet meniscus. Therefore, crystalline growth on silicon wafer or PMMA is not as rapid as on soda lemon glass and it would not greatly strike out of the liquid-vapor interface. 16‘02”

16’18”

16’30”

19’14”

(a) Csalt 10%, silicon wafer 31’44”

34’15”

32’11”

43’43”

(b) Csalt 10%, PMMA

Figure 10: The inward transport of crystal cube initially emerging near the contact line of droplet with Csalt at 10% on (a) silicon wafer and (b) PMMA plate. The scale bar indicates 0.5 mm. Different from the crystalline cube moving into the droplet center on silicon wafer and PMMA plate, large crystals form and stay near the perimeter at high Csalt on soda lemon glass over the droplet lifetime. The thin meniscus of the droplet led by high wettability and the strong pinning effect of the soda lemon glass, together with the fast development of precipitation, resulting in the wrecking of the liquid-vapor interface near the droplet rim. As shown in Figure 11 (b), in a short duration of 10 seconds from 13’05 s to 13’15 s, the precipitation develops large and conspicuously sabotages the liquid-vapor interface near the edge. As a result, the crystalline is no longer restricted within the solution area, and therefore it fails to be entrained inwards. Therefore, droplet wettability at the moment of precipitation is critical for the growth as well as motion of crystalline that whether it is subjected to capillary forces at the liquid-vapor interface. Droplet thickness at the onset of precipitation not only affects the locale of the formed crystals, but also determines the profile of precipitation. It is unexpectedly to find that the morphology of crystalline is distinctly different for a low and a high Csalt on soda lemon glass, despite that the droplets experienced similar lifetime pinning and possessed analogous supersaturation degrees at crystallization for various Csalt . As demonstrated in Figure 3 (c), at 2.5% and 5% of the Csalt the 21 ACS Paragon Plus Environment


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13’25”

13’50”

14’15”

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14’40”

(a) Csalt 25%, silicon wafer 13’05”

13’15”

13’25”

13’35”

(b) Csalt 25%, soda lemon glass Figure 11: The snapshots showing the intrusion of crystalline taken from side view of droplets with Csalt at 25% on (a) silicon wafer and (b) soda lemon glass. The scale bar indicates 1 mm. final deposit is comprised of exterior cracked layers of crystalline and several separated cubes in the interior region. Contrary to it, the large crystalline cubes are located near the outer region of the droplet at 20% and 25% Csalt . The formation of crystalline at 2.5% and 25% of Csalt as examples are presented in Figure 12 (a, b). It can be observed that the crystallization starts at the vicinity of the contact line for both Csalt . For Csalt at 2.5%, a thin branch-like layer of crystal is produced initially and meanwhile the remaining solution shrinks toward the droplet central area. From the moment on, crystalline cubes begin to form near the edge of the irregular solution. For Csalt at 25%, however, large crystalline cube appears at the very beginning of crystallization, and thin layers of salt are precipitated right before the droplet dry-out. From the inset of Figure 9 (a) it is known that precipitation takes place at similar supersaturation degrees for different Csalt , implying that the instant salt concentration of precipitation is analogous as well for different Csalt . Therefore, the different modes of crystallization is unlikely to be caused by different supersaturation. It is worth noticing that the droplet thickness at the onset of precipitation, represented by the dynamic contact angle, θp , is greatly dependent on Csalt , as 22 ACS Paragon Plus Environment

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14’01”

14’10”

14’15”

14’26”

15’51”

(a) Csalt 2.5% 13’08”

19’00”

25’00”

27’30”

29’30”

(b) Csalt 25% 25 (c) 20

15 p (°)

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10

5

0 0

5

10 15 Csalt (%)

20

25

Figure 12: The formation of crystalline pattern from the droplet with Csalt at (a) 2.5% and (b) 25% on soda lemon glass. (c) The contact angle at the onset of precipitation θp versus salt concentration Csalt . The scale bar indicates 1 mm.



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plotted in Figure 12 (c). At 2.5% and 5% of Csalt , θp is relatively small, ranging from 3◦ to 5◦ , in contrast to the counterpart above 15◦ and 21◦ for Csalt at 20% and 25%, respectively. This is because at a relatively low Csalt , precipitation emerges at the late stage of evaporation. As shown in Figure 9 (a), the normalized precipitation time tp is around 0.8, at which most solution has been evaporated and the droplet could be fairly thin reflected by the small dynamic contact angle. On the contrary, the droplets with Csalt at 20% and 25% have larger initial contact angles, as shown in Figure 8 (a). And since precipitation starts approximately in the middle of droplet lifetime, the dynamic contact angle could be at a moderate value. The corresponding thicker meniscus of the droplet provides a larger space for crystals to grow. Besides, once a crystalline is formed, the salt concentration of the neighboring region is reduced abruptly, and it could be lower than the saturation concentration Csat . The induced local gradient of salt ions could initiate transport of salt ions to the crystalline space. Therefore, the replenish of salt ions from nearby is essential for the continuous growth of crystalline, and which is expected to be more prompt if the cross-section area of the meniscus is greater. The thicker solution layer of the droplet with higher Csalt could allow a more responsively supply of the salt ions to the growth of crystalline cubes.

Conclusions In summary, we conducted an experiment to investigate the evaporative dynamics and crystalline patterns from sessile saline droplets on various substrates.

The substrate-dependent droplet

dynamics was found to have influence on the morphology and movement of salt crystalline after evaporation completed. On the silicon wafer and PMMA plate, the saline droplets exhibited unique dynamics that the dynamic contact angle increased for a continuous period in the middle of evaporation. Such an increase in contact angle was negatively dependent on the salt concentration. It was worth noticing that the onset of precipitation was fairly close to the end of contact angle enlargement when the contact angle reached its apex. The supersaturation degree at which precipitation occurred was larger at a higher salt concentration. The long stage of depinning


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on silicon wafer and PMMA plate provided relatively large space for the growth of crystal cube and meanwhile impelled it toward the drop center. On the contrary, the droplets had lifetime pinning on soda lemon glass. The supersaturation degree at which precipitation of salt emerged was insensitive to the salt concentration. The onset of precipitation could be stimulated by the contact line dynamics. The supersaturation was smaller for silicon wafer and PMMA on which droplets underwent depinning stages, while it was larger on soda lemon glass as the droplets kept lifetime pinning. The resulting crystalline deposit depended greatly on the salt concentration such that it was comprised of exterior cracked layers of salts and interior separated small cubes for low salt concentrations, while the large crystalline cubes were near the outer region for high salt concentrations. Such different morphologies of precipitation was attributed to the different space confined by the liquid-vapor and liquid-solid interfaces at the occurrence of precipitation. It is also found that large cubical crystals were subjected to capillary forces at liquid-vapor interface as long as the interface was insignificantly deformed.

Acknowledgement The authors acknowledge the support of A*STAR SERC A1783c0006.

References (1) Qazi, M. J.; Liefferink, R. W.; Schlegel, S. J.; Backus, E. H. G.; Bonn, D.; Shahidzadeh, N. Influence of Surfactants on Sodium Chloride Crystallization in Confinement. Langmuir 2017, 33, 4260-4268. (2) Wei, X.; Yang, J.; Li, Z.; Su, Y.; Wang, D. Comparison Investigation of the Effects of Ionic Surfactants on the Crystallization Behavior of Calcium Oxalate: From Cationic to Anionic Surfactant. Colloids Surf., A 2012, 401, 107-115. (3) Sammalkorpi, M.; Karttunen, M.; Haataja, M. Ionic Surfactant Aggregates in Saline



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Solutions: Sodium Dodecyl Sulfate (SDS) in the Presence of Excess Sodium Chloride (NaCl) or Calcium Chloride (CaCl2 ). J. Phys. Chem. B 2009, 113, 5863-5870. (4) Desarnaud, J.; Derluyn, H.; Carmeliet, J.; Bonn, D.; Shahidzadeh, N. Metastability Limit for the Nucleation of NaCl Crystals in Confinement. J. Phys. Chem. Lett. 2014, 5, 890-895. (5) Zhong, X.; Crivoi A.; Duan, F. Sessile Nanofluid Droplet Drying. Adv. Colloid and Interface Sci. 2015, 217, 13-30. (6) Feng, H.; Chong, K. S.; Ong, K. S.; Duan, F. Octagon to Square Wetting Area Transition of Water-Ethanol Droplets on a Micropyramid Substrate by Increasing Ethanol Concentration. Langmuir 2017, 33, 1147-1154. (7) Zhong, X.; Duan, F. Flow Regime and Deposition Pattern of Evaporating Binary Mixture Droplet Suspended with Particles. Eur. Phys. J. E 2016, 39, 16018-16025. (8) Anyfantakis, M.; Geng, Z.; Morel, M.; Rudiuk, S.; Baigl, D. Modulation of the Coffee-Ring Effect in Particle/Surfactant Mixtures: The Importance of Particle-Interface Interactions. Langmuir 2015, 31, 4113-4120. (9) Zhang, B.; Chen, X.; Dobnikar, J.; Wang Z.; Zhang, X. Spontaneous Wenzel to Cassie Dewetting Transition on Structured Surfaces. Phys. Rev. Fluids 2016, 1, 073904-4. (10) Xu, C.; Peng, S.; Qiao,G.; Zhang, X. Effects of the Molecular Structure of a Self-Assembled Monolayer on the Formation and Morphology of Surface Nanodroplets. Langmuir 2016, 32, 11197-11202. (11) Bahmani, L.; Neysari, M.; Maleki, M. The Study of Drying and Pattern Formation of Whole Human Blood Drops and the Effect of Thalassaemia and Neonatal Jaundice on the Patterns. Colloids Surf. A Physicochem. Eng. Asp. 2017, 513, 66-75. (12) Lee, H. H.; Fu, S. C.; Tso, C. Y.; Chao, C. Y. H. Study of Residue Patterns of Aqueous


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Nanofluid Droplets with Different Particle Sizes and Concentrations on Different Substrates. J. Heat Mass Transfer 2017, 105, 230-236. (13) Saxena, N.; Naik, T.; Paria, S. Organization of SiO2 and TiO2 Nanoparticles into Fractal Patterns on Glass Surface for the Generation of Superhydrophilicity. J. Phys. Chem. C 2017, 121, 2428-2436. (14) Li, H.; Luo, H.; Zhang, Z.; Li, Y.; Xiong, B.; Qiao, C.; Cao, X.; Wang, T.; He, Y.; Jing, G. Direct Bbservation of Nanoparticle Multiple-Ring Pattern Formation during Droplet Evaporation with Dark-Field Microscopy. Phys. Chem. Chem. Phys. 2016, 18, 13018-13025. (15) Chen, X.; Ma, R.; Li, J.; Hao, C.; Guo, W.; Luk, B. L.; Li, S. C.; Yao, S.; Wang, Z. Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects. Phys. Rew. Lett. 2012, 109, 116101. (16) Malvadkar, N. A.; Hancock M. J.; Sekeroglu, K.; Dressick W. J.; Demirel, M. C. An Engineered Anisotropic Nanofilm with Unidirectional Wetting Properties. Nature 2010, 9, 1023-1028. (17) Takhistov, P.; Chang, H. Complex Stain Morphologies. Ind. Eng. Chem. Res. 2002, 41, 62566269. (18) Townsend, E. R.; Enckevort, W. J. P. V.; Meijer, J. A. M.; Vlieg, E. Additive Enhanced Creeping of Sodium Chloride Crystals. Cryst. Growth Des. 2017, 17, 3107-3115. (19) Kaya, D.; Belyi, V. A.; Muthukumar, M. Pattern Formation in Drying Droplets of Polyelectrolyte and Salt. J. Chem. Phys. 2010, 133, 114905-2010. (20) Shahidzadeh, N.; Schut, M. F. L.; Desarnaud, J.; Prat, M.; Bonn, Da. Salt Stains from Evaporating Droplets. Sci Rep 2015, 5, 10335. (21) Shin B.; Moon M. W.; Kim H. Y. Rings, Igloos, and Pebbles of Salt Formed by Drying Saline Drops. Langmuir 2014, 30, 12837-12842. 27 ACS Paragon Plus Environment


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(22) Subra, S. Colloids Model for Atoms. Nat. Mater. 2006, 5, 253-254. (23) Shahidzadeh-Bonn, N.; Rafai, S.; Bonn, D.; Wegdam, G. Salt Crystallization during Evaporation: Impact of interfacial properties. Langmuir 2008, 24, 8599-8605. (24) Pinho, S. P.; Macedo, E. A. Solubility of NaCl, NaBr, and KCl in Water, Methanol, Ethanol, and Their Mixed Solvents. J. Chem. Eng. Data 2005, 50, 29-32. (25) Stalder, A. F.; Melchior, T.; Muller, M.; Sage, D.; Blu, T.; Unser, M. Low-Bond Axisymmetric Drop Shape Analysis for Surface Tension and Contact Angle Measurements of Sessile Drops. Colloids Surf., A 2010, 364, 72-81. (26) Hu, H.; Larson, G. R. Evaporation of a Sessile Droplet on a Substrate. J. Phys. Chem. B 2002, 106, 1334-1344.


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TOC Graphic. silicon&pmma

pin-to-depin

40

Precipitation glass

(°)

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pi nn in g

Precipitation

Precipitation 0

0

Csalt 20%

Csalt 5%

glass

2000

t (s)


Wettability Effect on Evaporation Dynamics and Crystalline Patterns of

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